Nanopyramid sized opto-electronic structure and method for manufacturing of same

ABSTRACT

Aspects of the invention provide methods and devices. In one embodiment, the invention relates to the growing of nitride semiconductors, applicable for a multitude of semiconductor devices such as diodes, LEDs and transistors. According to the method of the invention nitride semiconductor nanopyramids are grown utilizing a CVD based selective area growth technique. The nanopyramids are grown directly or as core-shell structures.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/030,145, filed Sep. 18, 2013, which claims benefit of U.S.Provisional Application No. 61/702,658, filed Sep. 18, 2012, the entiredisclosures of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to semiconductor devices based onsemiconductor nanopyramids and the method of producing same.

BACKGROUND

Nanostructures are promising building blocks for electronic andoptoelectronic semiconductor devices. The three-dimensional shape of thenanostructures can have challenges in device design. Differentcrystallographic planes can give different growth rate, materialcomposition, and doping.

SUMMARY

One embodiment provides a method of making a nanopyramid, comprisingforming a first conductivity type III-V semiconductor core or core seedusing CVD at a first temperature, first pressure and first III-V ratio,forming a first conductivity type III-V semiconductor shell enclosingthe core or the core seed of the first conductivity type semiconductorusing CVD at a second temperature, second pressure and second III-Vratio, and forming a first layer of a second conductivity typesemiconductor over the shell of the first conductivity typesemiconductor to form the nanopyramid. At least one of the firsttemperature, first pressure and first V-III ratio is different from atleast one of the second temperature, second pressure and second V-IIIratio.

Another embodiment provides a semiconductor light emitting devicecomprising a support and a plurality of nanopyramids arrayed on thesupport, each of the plurality of the nanopyramids comprising a firstconductivity type semiconductor core or core seed, a first conductivitytype semiconductor shell enclosing the core or the core seed of thefirst conductivity type semiconductor, and a first layer of a secondconductivity type semiconductor over the shell of the first conductivitytype semiconductor. The first conductivity type semiconductor shell andthe first layer of second conductivity type semiconductor are configuredto form a pn or pin junction that in operation provides an active regionfor light generation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a side cross sectional view of ananopyramid LED in accordance with an embodiment of the invention.

FIGS. 2a, 2b and 2c schematically illustrate side cross sectional viewsof nanopyramid LEDs with different shaped cores in accordance withalternative embodiments of the invention.

FIGS. 3a-3f are micrographs of nanopyramids in accordance withembodiments of the invention.

FIGS. 4a, 4b and 4c schematically illustrate side cross sectional viewsof steps in a method of making nanopyramid LEDs with an etched back tipmask in accordance with an embodiment of the invention.

FIG. 4d schematically illustrates a method of making nanopyramid LEDsusing angled deposition to form a tip mask in accordance with anotherembodiment of the invention.

FIG. 4e schematically illustrates a side cross sectional view of ananopyramid LED with the tip mask made by the method shown in FIG. 4d inaccordance with an embodiment of the invention.

FIGS. 5a and 5b schematically illustrate side cross sectional views ofmethods of making nanopyramid LEDs with a modified tip in accordancewith embodiments of the invention.

FIGS. 6a, 6b and 6c schematically illustrate side cross sectional viewsof steps in a method of making nanopyramid LEDs with a removed tip inaccordance with an embodiment of the invention.

FIGS. 7a, 7b and 7c schematically illustrate side cross sectional viewsof steps in an alternative method of making nanopyramid LEDs with aremoved tip in accordance with an embodiment of the invention. FIG. 7dillustrates an alternative nanopyramid LED to the one shown in FIG. 7 c.

FIGS. 8a, 8b, and 8c schematically illustrate side cross sectional viewsof a nanopyramid LED containing a shell having foot portion inaccordance with embodiments of the invention.

DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

The semiconductor device and method to produce such according to thepresent invention comprises at least one nitride semiconductornanopyramid, for example a GaN nanopyramid.

One embodiment of the invention is a nitride semiconductor nanopyramid,which in this context is essentially a pyramid shaped structure with abase width or diameter of about 1100 nm and a height range of about 1000nm. In some embodiments, the base diameter (or width and length fornon-circular bases) is about 100 nm to about 1500 nm and the height ofthe nanopyramids are from about 90 nm to about 1300 nm. In anotherembodiment, base widths range from 100 nm to several (e.g., 5) μm, suchas 100 nm to below 1 micron, and heights range from a few 100 nm toseveral (e.g., 10) μm. The nanopyramid is at its base epitaxiallyconnected to a support, which comprises of one or more epitaxial layers,for example a layer of GaN closest to the nanopyramid, located over asubstrate. The top of the nanopyramid can be a sharp point or a mesahaving a width smaller than the base. The nanopyramid core protrudesthrough an opening in a growth mask of for example SiNx. A semiconductordevice according to the invention typically comprises a plurality ofnanopyramids. The nanopyramids of the current invention preferably havea hexagonal or cubic base. Preferably, the nanopyramid core is coveredby a semiconductor shell, this shell being matched for conductivity withthe conductivity of the core and the shell being a pyramid shapedstructure. The shell preferably is covered by an active layer withquantum wells and one or more semiconductor layers, such semiconductorlayers being of a different conductivity than the core.

In one embodiment of the invention, an array of semiconductornanopyramids are used to form LED devices. Nanopyramid LEDs aretypically based on one or more pn- or p-i-n-junctions. The differencebetween a pn junction and a p-i-n-junction is that the latter has awider active region. The wider active region allows for a higherprobability of recombination in the i-region. Each nanopyramid comprisesa first conductivity type (e.g., n-type) nanopyramid core, a firstconductivity type (e.g., n-type) shell, and an enclosing secondconductivity type (e.g., p-type) layer for forming a pn or pin junctionthat in operation provides an active region for light generation. Whilethe first conductivity type of the core is described herein as an n-typesemiconductor core and the second conductivity type layer is describedherein as a p-type semiconductor layer, it should be understood thattheir conductivity types may be reversed.

FIG. 1 schematically illustrates the basis for a nanopyramid LEDstructure in accordance with some embodiments of the invention. Inprinciple, one single nanopyramid is enough for forming a nanopyramidLED, but due to the small size, nanopyramids are preferably arranged inarrays comprising hundreds, thousands, tens of thousands, or more, ofnanopyramids side by side to form the LED structure. For illustrativepurposes the individual nanopyramid LED devices will be described hereinas being made up from nanopyramids 1 having an n-type core 2, an n-typeshell 3, and at least one p-type layer 5, such as multiple p-type layers5 and 5′, at least partly enclosing the nanopyramid shell 3 and anintermediate active layer 4. In some embodiments, the nanopyramid core2, nanopyramid shell 3, the active layer 4 and the p-type layers 5 and5′ may be made up from a multitude of layers or segments. By controllinggrowth conditions the final geometry of the n-type core 2 can range fromelongated to narrow “pillar structures”. In some embodiments, the n-typeshell is enclosed with one p-type layer.

In alternative embodiments, only the core 2 may comprise a nanostructureby having a width or diameter below 1 micron, while the shell 3 may havea width or diameter above one micron.

FIGS. 2a-2c depict three embodiments of nanopyramids with differentshapes of the core 2. FIG. 2a is a nanopyramid with a conventional(e.g., semiconductor nanowire) core, FIG. 2b is a nanopyramid with apaused core (i.e., core formed by paused deposition) and FIG. 2c is ananopyramid with no core, i.e., the pyramids are gown directly on thegrowth mask 7 layer and on a core seed 2A located in an opening in thegrowth mask 7 which exposes the substrate (e.g., buffer layer 6 onsupporting substrate 8). For clarity, the thin active layer 4 is notshown and the p-type layers 5, 5′ are shown as a single layer 5 in FIGS.2a -2 c.

For nanopyramid fabrication, such as III-V nanopyramid fabrication, theIII-V semiconductors are of particular interest due to their propertiesfacilitating high speed and low power electronics. The nanopyramids cancomprise any semiconductor material, and suitable materials for thenanopyramid include but are not limited to: GaAs (p), InAs, Ge, ZnO,InN, GaInN, GaN, AlGaInN, BN, InP, InAsP, GaInP, InGaP:Si, InGaP:Zn,GaInAs, AlInP, GaAlInP, GaAlInAsP, GaInSb, InSb, Si. Possible donordopants for e.g. GaP are Si, Sn, Te, Se, S, etc, and acceptor dopantsfor the same material are Zn, Fe, Mg, Be, Cd, etc. It should be notedthat the nanopyramid technology makes it possible to use nitrides suchas GaN, InN and AlN, which facilitates fabrication of LEDs emittinglight in wavelength regions not easily accessible by conventionaltechnique. Other combinations of particular commercial interest include,but are not limited to GaAs, GaInP, GaAlInP, GaP systems. Typical dopinglevels range from 10¹⁸ to 10²⁰. A person skilled in the art is thoughfamiliar with these and other materials and realizes that othermaterials and material combinations are possible.

Preferred materials for nanopyramid LEDs are III-V semiconductors suchas a III-nitride semiconductor (e.g., GaN, AlInGaN, AlGaN and InGaN,etc.) or other semiconductors (e.g., InP, GaAs).

In a preferred embodiment, the nanopyramid is grown on a n-GaN bufferlayer 6, with a n-GaN core 2, a n-type GaN/InGaN shell 3, an activelayer 4 of InGaN with quantum wells, a layer of p-AlGaN 5, and a layerof p-GaN 5′. The shell 3 may comprise one or more subshells, such as aGaN subshell in physical contact with the core 2 and an InGaN subshellon the GaN subshell. Alternatively, the n-type core 2 and/or the n-typeshell 3 may comprise AlGaN or InAlGaN, in addition to or instead ofn-GaN and/or n-InGaN. The p-type semiconductor layer(s) (i.e., outershells) 5 and/or 5′ may also include p-AlInN or p-InAlN, in addition toor instead of p-AlGaN and/or p-GaN. Preferably, the active layer 4 ofInGaN with quantum wells is about 3-20 nm in thickness. The layer ofp-GaN 5′ may be coalesced, with or without voids in the coalesced p-GaNlayer, such as described in U.S. application Ser. No. 13/245,405, whichis incorporated herein in its entirety.

In order to function as a LED, the n-side and p-side of each nanopyramid1 has to be contacted.

As mentioned above, the first electrode may comprise an n-electrodelayer, e.g., a buffer layer 6, such as a buffer layer comprising galliumnitride or aluminum gallium nitride, from which the nanopyramid core wasgrown during production of the array of nanopyramids. The secondelectrode may comprise a p-electrode layer that is electricallyconnected to the p-type layers, such as a transparent conductive oxidelayer, e.g., Indium Tin Oxide (ITO). The second electrode may be formedby any suitable method.

The fabrication methods described in U.S. Pat. No. 7,829,443, to Seifertet al., incorporated herein by reference, would be suitable for growthof the nanopyramids described herein, however it should be noted thatthe invention is not so limited. In one embodiment, the invention is ananopyramid core to grow semiconductor shell layers on the cores to forma core-shell nanopyramid. For example, in alternative embodiments, onlythe core may constitute the nanostructure (e.g., nanopyramid) while theshell may optionally have dimensions which are larger than typicalnanopyramid shells. Furthermore, the device can be shaped to includemany facets, and the area ratio between different types of facets may becontrolled.

The use of sequential (e.g., shell) layers may result in the finalindividual device (e.g., a pn or pin device) having a pyramid shape(i.e., narrower at the top or tip and wider at the base) with cubic orhexagonal or other polygonal cross section perpendicular to the longaxis of the device. Thus, the individual devices with the completedshells may have various sizes. For example, the sizes may vary, withbase widths ranging from 100 nm to several (e.g., 5) μm, such as 100 nmto below 1 micron, and heights ranging from a few 100 nm to several(e.g., 10) μm.

FIG. 1 illustrates an exemplary structure that provides a support forthe nanopyramids. By growing the nanopyramids 1 on a growth substrate 8,optionally using a growth mask, or dielectric masking layer 7 (e.g., anitride layer, such as silicon nitride dielectric masking layer) todefine the position and determine the bottom interface area of thenanopyramids 1, the substrate 8 functions as a carrier for thenanopyramids 1 that protrude from the substrate 8, at least duringprocessing. The bottom interface area of the nanopyramids comprises thearea of the core 2 inside each opening in the dielectric masking layer7. The substrate 8 may comprise different materials such as III-V orII-VI semiconductors, Si, Ge, Al₂O₃, SiC, Quartz, glass, etc., asdiscussed in Swedish patent application SE 1050700-2 (assigned to GLOAB), which is incorporated by reference herein in its entirety. Othersuitable materials for the substrate include, but are not limited to:GaAs, GaP, GaP:Zn, GaAs, InAs, InP, GaN, GaSb, ZnO, InSb, SOI(silicon-on-insulator), CdS, ZnSe, CdTe. In one embodiment, thenanopyramids 1 are grown directly on the growth substrate 8.

In embodiments in which a dielectric masking (growth mask) layer isused, the growth mask 7 may be patterned by photolithography to defineopenings for the nanopyramid growth, as described for example in U.S.Pat. No. 7,829,443, incorporated herein by reference in its entirety. Inone embodiment, the nanopyramids are grouped in an n-pad area, anon-active area, a LED area (i.e., the area which emits light) and ap-pad area. However, embodiments of the invention are not limited tothis. For example the p-pad area may be arranged on top of thenanopyramids forming the light emitting part of the nanopyramid LEDstructure, whereby the p-pad area and the LED area coincide, asdescribed in PCT International Application Publication Number WO2010/014032 A1 to Konsek, et al., published Feb. 4, 2010 andincorporated herein by reference in its entirety.

Preferably, the substrate 8 is also adapted to function as a currenttransport layer connecting to the n-side of each nanopyramid 1. This canbe accomplished by having a substrate 8 that comprises a buffer layer 6arranged on the surface of the substrate 8 facing the nanopyramids 1, asshown in FIG. 1, by way of example a III-nitride layer, such as a GaNand/or AlGaN buffer layer 6 on a Si or Al₂O₃ substrate 8. The bufferlayer 6 is usually matched to the desired nanopyramid material, and thusfunctions as a growth template in the fabrication process. For an n-typecore 2, the buffer layer 6 is preferably also doped n-type. The bufferlayer 6 may comprise a single layer (e.g., GaN), several sublayers(e.g., GaN and AlGaN) or a graded layer which is graded from high Alcontent AlGaN to a lower Al content AlGaN or GaN. The nanopyramids cancomprise any semiconductor material, but for nanopyramid LEDs III-Vsemiconductors such as a III-nitride semiconductor (e.g., GaN, AlInGaN,AlGaN and InGaN, etc.) or other semiconductors (e.g., InP, GaAs) areusually preferred. The growth of nanopyramids can be achieved byutilizing methods described in the U.S. Pat. Nos. 7,396,696, 7,335,908,and 7,829,443, and WO201014032, WO2008048704 and WO 2007102781, all ofwhich are incorporated by reference in their entirety herein.

It should be noted that the nanopyramid 1 may comprise several differentmaterials (e.g., GaN core, InGaN active layer and InGaN shell having adifferent In to Ga ratio than the active layer). In general thesubstrate 8 and/or the buffer layer 6 are referred to herein as asupport or a support layer for the nanopyramids. In certain embodiments,a conductive layer (e.g., a mirror or transparent contact) may be usedas a support instead of or in addition to the substrate 8 and/or thebuffer layer 6. Thus, the term “support layer” or “support” may includeany one or more of these elements. The buffer layer 6 provides astructure for contacting the n-side of the nanopyramids 1.

The above description is exemplary embodiments of a LED structure;however, it will be appreciated that any suitable nanopyramid LEDstructure or other suitable nanopyramid structure may also be used inthe methods and compositions, with any necessary modifications as willbe apparent to one of skill in the art, without departing from theinvention.

The method of growing nitride semiconductor nanopyramids according tothe embodiments of the invention preferably utilizes a CVD basedselective area growth technique. A nitrogen source and a metal-organicsource are present during the nanopyramid growth step and at least thenitrogen source flow rate is continuous during the nanopyramid growthstep.

In one embodiment, the nanopyramids of the present invention are grownusing metal organic chemical vapor deposition (MOCVD) processes. OtherCVD and hydride vapor phase epitaxy (HVPE) based processes withmodifications are also suitable.

One embodiment of the method comprises the steps of: a) providing agrowth mask on a support. The support is for example GaN (e.g., n-GaNbuffer layer 6 over supporting substrate 8) and the growth mask 7 is adielectric such as of SiN_(x) or SiO_(x), b) produce openings in thegrowth mask. The openings are preferably well controlled, both inregards to their diameter and their relative positioning. Severaltechniques known in the art can be used for the procedure including, butnot limited to electron beam lithography (EBL), nanoimprint lithography,optical lithography and reactive ion etching (RIE) or wet chemicaletching methods. Preferably the openings are about 100 nm to about 200nm in diameter and pitched about 0.5 to about 5 μm apart. The openingsdefine the position and the diameter of the nanopyramids to be produced.c) Performing nanopyramid growth by a CVD based process wherein theprecursor source flows are continuous. The precursor source flow ratesare adjusted to achieve a low supersaturation in the growth zone.

Nanopyramids fabricated with the method according to the invention areillustrated in the SEM images of FIGS. 3a-f . On the starting substratea layer of SiN_(x) (about 30-100 nm in thickness) was deposited byPECVD. In a subsequent step, arrays of dot-patterned GaN openings (about100 nm in diameter) were made by electron beam lithography, EBL, andreactive ion etching, RIE. The pitch between the openings was ranged asabout 0.5-3.2 μm, giving a growth mask that defines both the base sizeand the positions of the nanopyramids. Then, the as-processed samplesare inserted into a horizontal MOCVD chamber to grow GaN nanopyramids.FIGS. 3a-f illustrate the different types of nanopyramids formed withthe three types of cores. FIGS. 3a and 3b are respective lower andhigher magnification micrographs of nanopyramids with a conventionalcore, FIGS. 3c and 3d are respective lower and higher magnificationmicrographs of nanopyramids with a paused core, and FIGS. 3e and 3f arerespective lower and higher magnification micrographs of nanopyramidswith no core, i.e., the pyramids are grown directly on the growth masklayer through the substrate.

In one embodiment, the conventional core is grown at high temperature,low pressure, and extremely low V/III ratio—about 5 to about 10. Thisgrowth is continued when both group III and V precursors are suppliedduring growth. Once a desirable height is reached, growth conditions areswitched to lower temperature, higher pressure, and much higher V/IIIratio (about 10,000 to about 15,000). Preferred growth conditions aredepicted in TABLE 1.

TABLE 1 PREFERRED GROWTH CONDITIONS FOR CONVENTIONAL CORE TemperaturePressure V/III ratio n-core About 1000 to About 50 to About 5 to about1200° C. about 100 Torr about 10 n- shell About 800 to About 100 toAbout 10,000 to about 1000° C. about 500 Torr about 15,000

The paused (e.g., interrupted) core is grown in more than one step. Thegrowth conditions are similar to the conditions used to grow aconventional core, however there is a pause during which themetal-organic precursor (e.g., a Group III metal organic precursor, suchas TMG, TEG, etc.) is no longer flowing into the CVD growth chamber, butthe nitrogen containing precursor (e.g., ammonia) flow is increased (atleast an order of magnitude higher). The nucleation and the first stepis similar to the growth conditions for the conventional core. Once thecore is nucleated and the template's hole is filled, growth isinterrupted and the nitridation step is conducted at high ammonia flow.After nitridation, the core growth is resumed using the conditions forgrowth of a conventional core. In this embodiment of the method forgrowing the paused core, during growth interruption the ammonia flow isincreased significantly and annealing at high ammonia flow is applied.Such plural alternating growth and nitridation (time about 60 to about180 seconds) steps are performed until desirable height of core isreached. Preferred growth conditions are depicted in TABLE 2.

TABLE 2 PREFERRED GROWTH CONDITIONS FOR CONVENTIONAL CORE TemperaturePressure V/III ratio n-core About 1000 to About 50 to Continuous growthwith about 1200° C. about 100 Torr a V/III ratio of about 5 to about 10;followed by nitridation (growth interruption) during which themetal-organic precursor is not flowing and the nitrogen containingprecursor (e.g., ammonia) flow is increased to about 15 000 to about20000; after nitridation, metal-organic precursor flow is restarted andthe ammonia flow is reduced (5 to 10) such that the growth resumed;These alternating growth and nitridation steps are repeated plural timesuntil desirable height of core is reached. n- shell About 800 to About100 to About 10,000 to about 1000° C. about 500 Torr about 15,000

The direct growth core is achieved under growth conditions of lowertemperature, higher pressure, and higher VIII ratio right after corenucleation (e.g., the core nucleus is present in the opening in thegrowth mask). This is a preferred embodiment of a method for lowtemperature growth of InGaN pyramids. This enables growth of InGaNpyramids with high In, in a constant trimethyl indium flow providing auniform In content through the pyramid structure. In another approach,graded In composition is applied when the pyramid is nucleated as GaNand then the In composition is gradually increased from 0 to ˜0.6In/In+Ga. InGaN can also be applied as an underlayer once the GaNpyramid is grown.

In the alternative embodiments illustrated in FIGS. 4 through 7, thenanopyramid structure is modified to decrease the amount of leakagecurrent at the tip of the nanopyramid. In these embodiments, at leastone feature (e.g., additional component or structural modification)which reduces or eliminates the leakage current at the tip of the LEDdevice is added to the nanopyramid device.

In one exemplary embodiment shown in FIGS. 4a-4c , an insulating tipmask is created to selectively insulate the tip of the nanopyramid 1. Aninsulating layer 41 is deposited over a nanopyramid structure (e.g.,over p-GaN layer 5′). The insulating layer may be SiO_(x) (siliconoxide, e.g., SiO₂) or other suitable insulating material, such as Al₂O₃,intrinsic ZnO, SiN_(x) (e.g., Si₃N₄), HfO₂, or the like, as known in theart. Other high resistivity materials, such as intrinsic III-nitridesemiconductor materials (e.g., i-GaN, i-AlGaN, etc.) may also be used asthe layer 41. As shown in FIG. 4a , the insulating layer 41, e.g.,SiO_(x), such as SiO₂, is deposited over the nanopyramid structure byany suitable method, such as sputter deposition. Because of the highaspect ratio of the nanopyramids, more insulating material is depositedon the tip (i.e., top) of the nanopyramid than on the lower portion ofthe sidewalls (e.g., sidewalls of layer 5′).

The insulating layer 41 can then be treated by any suitable method,e.g., any suitable etching method, such as anisotropic (non-directional)etch, to selectively remove the insulating layer, and the conditionscontrolled, e.g., the time of the etch, to allow removal of theinsulating layer such that the thinner insulating layer on the sidewalls5′ is removed and the p-GaN layer 5′ is re-exposed below the tip, butthe tip is still covered with a remaining portion of the thickerinsulating layer 47 (i.e., the tip mask), as shown in FIG. 4b . For agiven method and nanopyramid structure, the time of etch that producesoptimal results may be determined, e.g., either by calculation or byempirical observation, or both, as known in the art. The result is aninsulating layer 47 of, e.g., SiO_(x), such as SiO₂, on the top part ofthe nanopyramid, i.e., on the tip with no insulator or lower insulatorthickness on the lower parts of the sidewalls 5′.

A p-side electrode 49, is then formed over the tip mask 47 and theexposed portions of the lower parts of the p-GaN layer 5′. The electrodemay comprise any suitable conductive material, e.g., a transparentconductive oxide, such as ITO. The electrode 49 electrically contactsthe lower parts of the p-GaN layer 5′ but is electrically isolated fromthe tip of the p-GaN layer 5′ by the tip mask 47.

In another embodiment shown in FIGS. 4d and 4e , the tip mask isselectively formed on the tips of the nanopyramids by angled depositionrather than by deposition and etchback described above. The angleddeposition method for treating a LED structure that comprises an arrayof nanopyramids on a planar support comprises providing the insulatingtip mask material at a source and causing it to move to the array alonga line, where (i) the angle between the line followed by the materialand the plane of the support is less than 90° when measured from thecenter of the support; and (ii) the material is capable of rendering atip portion of the nanopyramids nonconductive or less conductivecompared to before being treated by the material. The LED structure istypically rotated about an axis, or several axes, to expose differentportions of the structure to the material produced at the source; inthese cases, the center of the support is considered to be theintersection of the axis of rotation with the support.

In certain of these embodiments, the material is an insulator, andrenders the tip portion of the nanopyramids nonconductive or lessconductive by coating the tip portion. Suitable dielectric materials foruse as insulators include Al₂O₃, i-ZnO, SiO₂, TiO₂, SiN_(x), and HfO₂.In certain embodiments, the material is Al₂O₃.

In certain of these embodiments, the material reacts with the tipportion of the nanopyramid to render the tip portion nonconductive orless conductive. For example, in certain embodiments the materialcomprises implanted H₂ ⁺, or at least one of noble gas and oxygenplasma.

FIG. 4d illustrates a general scheme for angled deposition of aninsulating material on the nanopyramid LED structure. Al₂O₃ is shown asthe insulator but any suitable insulator, such as dielectric materialsuch as i-ZnO, SiO₂, SiN_(x), HfO₂, TiO₂, and the like, may be used. Thenanopyramid LED structure is held in place so that the support is in aplane as shown, with nanopyramids 1 facing perpendicular to the plane.The nanopyramid LED structure can be considered to have outernanopyramids and inner nanopyramids, where the outer nanopyramids arethe last row of nanopyramids in the nanopyramid LED structure. A sourceof insulating material 9, e.g., Al₂O₃, is positioned such that thesource is at an angle α from the plane of the nanopyramid LED structure,where a is measured as the angle between a line from the center of thesource of insulating material to the center of the LED structure and theplane of the nanopyramid LED structure. If the shape of the LEDstructure is irregular, the center is considered to be the point atwhich the axis of rotation intersects the plane of the LED structure.The axis of rotation may or may not coincide with the center of thesubstrate 8. There also may be multiple axes of rotation. Energy issupplied to the source of the insulating material so as to causemolecules or portions of the material to enter the vapor phase, e.g., byelectron beam evaporation or other suitable technique, e.g., sputterdeposition. Molecules or other discrete units of the insulating materialmove out from the source material and those that intersect thenanopyramid LED structure are deposited on the tips and on the sidewallsof the nanopyramids 1. The nanopyramid LED structure is rotated aboutits center as the deposition occurs on an axis that intersects the LEDstructure at a point of intersection 10. Sidewalls of outer nanopyramidsmay be coated completely, but inner nanopyramids are shadowed from thedeposition and the sidewalls of inner nanopyramids are partially coated,the portion of the nanopyramid that is coated being determined by theangle α; generally, the smaller α is, less of the sidewall of innernanopyramids will be coated. The nanopyramid LED structure may includeopen spaces or isolated nanopyramids, and in these cases thenanopyramids on the edge of the open space or the isolated nanopyramidsmay have part or all of their sidewalls coated, depending on the size ofthe open space or degree of isolation and on the angle α.

An exemplary end result is shown in FIG. 4e . In this case, angleddeposition of the insulating layer (e.g., Al₂O₃) selectively forms tipmasks 47A on the tips of the interior nanopyramids 1, while leaving thelower parts of the sidewalls of layer 5′ exposed.

A transparent conductive oxide (TCO) layer, such as an ITO layer 49, isthen deposited over the structure by any suitable method, e.g., sputterdeposition, to make electrical contact with the exposed sidewalls of thep-GaN layer 5′ of the nanopyramids that have not been coated with theinsulating material of the tip mask 47A. This forms a p-side electrode49.

Then, laser ablation or patterning and etching is performed to exposethe n-GaN buffer layer 6 in certain areas. N-side contact conductivelayer(s) are formed on the exposed buffer 6 layer to provide an n-sidecontact 43 which is electrically connected to the n-type portions 2, 3of the nanopyramids via the buffer layer 6. A p-side contact 45 is thenformed on the p-side electrode 49. The insulating tip mask 47Aselectively formed by angled deposition on the tip of the nanopyramidsacts to prevent or greatly reduce current leakage through the tips ofthe nanopyramids so that current is directed to the exposed areas of thesidewalls.

In another embodiment, shown in FIGS. 5A and 5B, a ²H⁺ implant is usedto modify the tip parts of the nanopyramids 1 in a nanopyramid LEDstructure. In this embodiment, the implant modifies the tip of the p-GaNlayer 5′ to form the higher resistivity tip mask 47B in rather than onthe tip of layer 5′.

As shown in FIG. 5A, the tip mask 47B is formed by angled H₂ ⁺implantation schematically shown by arrows 52A. The principle is thesame as for the deposition of an insulating material 47A, discussedabove, but in the case of hydrogen implantation, H₂ ⁺ ions are directedto the substrate. The hydrogen molecules or atoms are implanted to ashallow depth in the tips of the layer 5′ of the nanopyramids 1. Thedepth may be controlled by the energy of the implant and the angle atwhich the species impacts the surface. The hydrogen passivates activeacceptor and or donor dopants in the semiconductor (e.g., p-GaN 5′),thus rendering it less or non-conductive. As with angled deposition ofan insulating material, the portion of the sidewalls of interiornanopyramids exposed to the hydrogen implant depends on the angle α;outer nanopyramids sidewalls may be entirely exposed.

In another embodiment shown in FIG. 5C, rather than using an angledimplantation, straight line implantation of hydrogen ions schematicallyshown by arrows 52B may be used instead to form the tip mask 47B. Inthis embodiment, a sacrificial protective layer 54 may be formed betweenthe nanopyramids 1 to cover the lower parts of the sidewalls but toexpose the tips of the nanopyramids. The sacrificial protective layer 54may comprise a spin-on dielectric layer, such as a spin-on glass oranother suitable material, such as photoresist or another polymericmaterial, which is selectively formed between the nanopyramids 1 but notover the top of the nanopyramids 1. The sacrificial protective layer 54may be removed by selective etching or another suitable method (e.g.,ashing if layer 54 is a photoresist) after the implantation 52B iscompleted.

In another embodiment, plasma is used to damage the tips. The plasma canbe of various suitable species, such as Ar (or another noble gas) or O₂.The plasma 52B is directed onto the tips of the nanopyramids 1, whilethe lower parts of the nanopyramids are covered by a protective layer54, such as photoresist. Because the nanopyramid tips are damaged fromthe plasma, electrical current preferentially passes through the lowerparts of the side walls.

In one embodiment, Ar⁺ is used for treatment to increase the resistivityand contact resistance of p-type GaN. By exposing p-type GaN toenergetic Ar⁺ ions, the sheet resistance of the GaN is increased. Thecontact resistance of ITO 49 to p-GaN 5′ is also increased on p-GaNsurfaces treated with Ar⁺ compared to those that are not. The increasein p-GaN sheet resistance and/or contact resistance is sufficient todecrease leakage currents originating in the p-GaN. Substratescontaining nanopyramids 1 are placed in a chamber containing argon at apressure of 100 mTorr, and a plasma is struck with a power of 100 Watts,Ar ions bombard the exposed tips of the nanopyramids 1. In general, thehydrogen implantation and/or the plasma damage forms a higherresistivity III-nitride (e.g., p-GaN) hydrogen implanted or plasmadamaged tip region having a higher resistivity than semiconductorsidewalls 5′ of the nanopyramids.

In another embodiment of the invention shown in FIGS. 6a-6c , the tipsof the p-GaN layer of the nanopyramids are removed. In a preferredembodiment, the nanopyramid tips are selectively removed leaving thelower parts of layer 5′ undisturbed. This technique of removal of thetip can be used in combination with a deposited tip mask 47 (e.g., Al₂O₃film) and/or by increasing the contact resistance and resistivity of thep-GaN by forming the tip mask 47A or 47B in the p-GaN layer 5′ tip asdescribed above.

As shown in FIG. 6a , the sacrificial protective layer 54 describedabove is formed between the nanopyramids such that the tips 65 of thenanopyramids are exposed in the protective layer 54 while the lowerparts of the sidewalls 5′ are covered by the sacrificial protectivelayer 54. Then, as shown in FIG. 6b , the exposed tips 65 of thenanopyramids are removed. The tips 65 may be removed by selectiveetching which etches the tips preferentially to the material of theprotective layer 54 or by chemical mechanical polishing such that thetop surface of the nanopyramids is even with the top surface of theprotective layer 54. This forms a truncated nanopyramid 1A (i.e., ananopyramid having a trapezoid shape, e.g. a convex quadrilateral withone pair of parallel sides).

Then, as shown in FIG. 6c , the sacrificial protective layer 54 isremoved and the p-side electrode 49 is formed in contact with theexposed sidewalls of the p-GaN layer 5′ of the nanopyramid 1 a. Theelectrode 49 may be formed by depositing a TCO layer over and betweenthe nanopyramids 1 a followed by planarizing the TCO layer by chemicalmechanical polishing or etch back to expose the upper surface of thenanopyramids 1 a. An insulating layer 68 may then be formed over theelectrode 49 and the nanopyramids 1 a.

In an alternative embodiment, the p-side electrode 49 may be formedinstead of the sacrificial protective layer 54 in FIG. 6a and be used toprotect the lower sidewalls 5′ of the nanopyramids during the tip 65removal in FIG. 6b . The electrode 49 then remains in the device asshown in FIG. 6 c.

In an alternative embodiment shown in FIG. 7, rather than removing thetip 65 of the completed nanopyramid (e.g., the tip of the p-typelayer(s) 5 and/or 5′), the tip 75 of the n-type shell 3 of thenanopyramid 1 is removed before the formation of the active region 4 andthe p-type region(s) 5 and/or 5′. As shown in FIG. 7a , the pyramidshaped n-type shell 3 is formed as described above with respect to FIG.2a, 2b or 2 c. Then, the tip 75 of the shell 3 is removed by etching orchemical mechanical polishing. If the desired, the sacrificialprotective layer 54 described above with respect to FIGS. 6a and 6b maybe used during the tip 75 removal. This forms a truncated pyramid (i.e.,a pyramid having a trapezoid shape) shaped n-type shell 3 a with a flatupper surface, as shown in FIG. 7 b.

Then, as shown in FIG. 7c , the active region 4 a and one or more p-typeshells 5 a described above are conformally formed on the n-type shell 3a. A p-side electrode 49 is then formed over the device. This forms atruncated nanopyramid 1 b shaped LED device.

In an alternative embodiment shown in FIG. 7d the active region b 4 a isformed conformally over the truncated pyramid shaped n-type shell 3 a.Then, one or more p-type shells 5 b are formed over the active layer. Inthis embodiment, the p-type shell(s) 5 b are not conformally depositedand have a pyramid shape with a relatively sharp tip. The p-sideelectrode 49 is then formed over the device. This forms a nanopyramidLED device 1 c having a pyramid shape, but which includes an internaln-type shell 3 a and preferably an active region 4 a which have atruncated pyramid shape. The active region thus has c-plane top surface.

In another embodiment, at least one feature (e.g., additional componentor structural modification) which reduces or eliminates the leakagecurrent at the base of the LED device is added to the nanopyramiddevice.

LED radiation (e.g., visible light, UV or IR) emission can be reduced byunintentional leakage of current in the diode. One source of leakage hasbeen identified by its physical location at the “foot” or base region 80of the nanopyramid LED, as shown in FIG. 8a . The foot or base region 80is located over the mask 7 between the p-type layer(s) or shell(s), 5′and the n-type interior shell 3/nanopyramid core 2. This base leakagemanifests itself by relatively high current, which is below the “turnon” of the diode.

One embodiment of the invention provides a structure and method toreduce this leakage current by decreasing contamination of the maskinglayer 7 during III-nitride semiconductor layer growth.

Without wishing to be bound by a particular theory, it is believed thatdecomposition of the silicon nitride masking layer 7 can occur duringthe growth of high temperature (e.g., at a temperature of at least 850C) of binary, ternary or quaternary III-nitride semiconductor layers,such as (Al)(In)GaN layers (i.e., AlGaN, InGaN and/or InAlGaN activeand/or shell layers). The masking layer 7 is continuously exposed duringthe growth of (Al)(In)GaN layers because (Al)(In)GaN easily desorbs fromthe silicon nitride masking layer 7 surface at the high temperature.Contamination of the silicon nitride masking layer from the III-nitridesemiconductor growth can potentially create or worsen a leakage currentfor the device.

In one embodiment, the base leakage may be reduced by the formation of asemiconductor foot 13 portion of the AlGaN p-type layer or shell 5 thatextends away from the shell over the surface of the mask 7 exposedbetween the nanopyramids 1, as shown in FIG. 8a . The foot 13 protectsthe masking layer 7 from contamination during the growth of the otherIII-nitride layers (e.g., 5′) and thus may reduce the leakage current.

If the nanopyramid core 2 and inner shell 3 comprise an n-typeIII-nitride semiconductor, such as n-type GaN, then the shell sublayers5, 5′ may comprise p-type III-nitride semiconductor sublayers with adifferent composition. For example, the inner shell 5 may comprisep-AlGaN and the outer shell 5′ may comprise p-GaN.

In an embodiment, a polycrystalline p-AlGaN foot 13 portion of the shellor layer 5 may be formed over the exposed portion of the silicon nitridemask 7 between the inner shells 3 and outer shell 5′ during the growthof the shell 5. The foot 13 portion may be formed, for example, byselecting a growth temperature (for example a growth temperature lowerthan 850 C) and/or CVD precursor gas flow ratio (for example a ratio ofthe Al containing gas to the ammonia gas, and/or or a ratio of nitrogento ammonia) that results in the formation of the polycrystalline p-AlGaNfoot 13 portion. Other growth conditions or parameters may also be used.

The resistivity of the AlGaN layer increases with increasing Alconcentration. Thus, the AlGaN foot 13 portion containing at least 5atomic percent aluminum has a relatively high resistivity to reduceleakage current. The foot 13 portion also provides a chemicallyresistant layer to protect the masking layer 7 during device processingand may also behave as a getter for oxygen impurities due to the strongatomic bond of oxygen with Al.

Specifically, AlGaN foot 13 portion may be deposited directly on thesilicon nitride masking layer 7. The combination of the shell whichcontains a foot 13 portion can also be referred to as a foot blockinglayer.

The AlGaN layer can be deposited anytime during the device growth. Forexample, the AlGaN layer may comprise the inner p-type shell 5 whichforms the foot 13 portion during growth on the masking layer 7, as shownin FIG. 8 a.

In another embodiment, the foot 13 portion is formed prior to the activeregion 4 formation. In this embodiment, the AlGaN foot 13 portion maycomprise a portion of one of the inner n-type shells located under theactive region 4. For example, the n-type shell 3 may comprise an outern-AlGaN subshell having a foot 13 portion and an inner n-GaN or InGaNsubshell, as shown in FIG. 8b . In one embodiment, the n-AlGaN shell 3may contain greater than 5 atomic % Al, such as at least 10 atomic % Al.

In another embodiment shown in FIG. 8c , an optional n-AlGaN shell 83(e.g., Si-doped n-AlGaN) having more than 5 atomic percent Al is formedaround the nanopyramid core 2. The AlGaN shell 83 growth step also formsan n-AlGaN foot 13 portion on the masking layer 7. The n-type shell 3,active region 4 (e.g., GaN/InGaN quantum wells), and the outer shells(which may comprise p-AlGaN shell 5 and p-GaN shell 5′) are then formedon the foot 13 region.

In another embodiment, two foot portions may disposed on the dielectricmasking layer 7. For example, shell 5 and one of the shells 3 or 83 maycontain separate foot 13 portions which are overlaid on each other. Insummary, the foot 13 portion may be formed prior to forming the activeregion 4, as shown in FIG. 8b or 8 c and/or after forming the activeregion 4, as shown in FIG. 8a . Alternatively, the LED device maycomprise multiple foot blocking layers having the same or oppositeconductivity type formed before and/or after forming the active region.

The foot blocking layer preferably comprises one or more of Al, Ga, In,B, Si, Mg, N, such as a III-nitride semiconductor, where Si and/or Mgare added as n-type and p-type dopants. In one embodiment, the footblocking layer comprises AlGaN and has a polycrystalline structure inthe regions 13 formed on the dielectric masking layer 8. Regions (e.g.,shells 3, 5, 83) formed on a single crystalline layer, core or shell,may have a single crystalline structure. The foot blocking layer may bedeposited by any one of several methods, such as metalorganic chemicalvapor deposition (MOCVD), molecular beam epitaxy (MBE), plasma-enhancedchemical vapor deposition (PECVD), plasma-assisted molecular beamepitaxy (PAMBE), or reactive sputtering deposition. Other methods may beused as well. In a preferred embodiment, the foot blocking layercomprises polycrystalline AlGaN foot 13 portion on the dielectricmasking layer 7 and a crystalline AlGaN shell 3, 5 or 83 formed byMOCVD.

The thickness of the AlGaN foot 13 portion on the silicon nitridemasking layer 7 is preferably greater than 10 nm to enable good surfacecoverage but less than 100 nm to not interfere with the outer shell 5′growth. For example, the AlGaN foot 13 portion may be 20-80 nm thick andcontains at least 10 atomic percent Al, such as 10-15 atomic percent.However, other thicknesses may be used.

While the inner shell(s) 3 are described above as being n-type and theouter shell(s) 5, 5′ are described as being p-type, it should beunderstood that their conductivity type may be reversed such that theinner shell(s) are p-type and the outer shell(s) are n-type. In thiscase, the buffer layer 6 is preferably p-type. The nanopyramids, such asthe Group III-nitride (e.g., GaN, GaAlN, InGaN, InGaAlN, etc.,nanopyramids, according to the embodiments of the invention can be usedin many different applications. Applications of particular interestinclude electronic, optical and optoelectronic devices including, butnot limited to: diodes, light emitting diodes (LEDs), laser diodes,transistors, photonic crystals and detectors. The nanopyramids can alsobe used as structural building blocks used to, for example, formcoalesce continuous layers of GaN. The LED devices are used above as anon limiting example. As appreciated by the person skilled in the arttransistors and other electronic devices can be fabricated in the samemanner.

All publications and patents cited in this specification are hereinincorporated by reference as if each individual publication or patentwere specifically and individually indicated to be incorporated byreference and are incorporated herein by reference to disclose anddescribe the methods and/or materials in connection with which thepublications are cited. The citation of any publication is for itsdisclosure prior to the filing date and should not be construed as anadmission that the present invention is not entitled to antedate suchpublication by virtue of prior invention. Further, the dates ofpublication provided may be different from the actual publication dateswhich may need to be independently confirmed.

The foregoing method descriptions are provided merely as illustrativeexamples and are not intended to require or imply that the steps of thevarious embodiments must be performed in the order presented. As will beappreciated by one of skill in the art the order of steps in theforegoing embodiments may be performed in any order. Words such as“thereafter,” “then,” “next,” etc. are not necessarily intended to limitthe order of the steps; these words may be used to guide the readerthrough the description of the methods. Further, any reference to claimelements in the singular, for example, using the articles “a,” “an” or“the” is not to be construed as limiting the element to the singular.

Further, any step or component of any embodiment described herein can beused in any other embodiment.

The preceding description of the disclosed aspects is provided to enableany person skilled in the art to make or use the present invention.Various modifications to these aspects will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other aspects without departing from the scope of theinvention. Thus, the present invention is not intended to be limited tothe aspects shown herein but is to be accorded the widest scopeconsistent with the principles and novel features disclosed herein.

The invention claimed is:
 1. A method of making a nanopyramid,comprising: forming a first conductivity type III-V semiconductor coreor core seed using CVD at a first temperature, first pressure and firstV-III ratio; forming a first conductivity type III-V semiconductor shellenclosing the core or the core seed of the first conductivity typesemiconductor using CVD at a second temperature, second pressure andsecond V-III ratio; and forming a first layer of a second conductivitytype semiconductor over the shell of the first conductivity typesemiconductor to form the nanopyramid; wherein at least one of the firsttemperature, first pressure and first V-III ratio is different from atleast one of the second temperature, second pressure and second V-IIIratio; wherein the nanopyramid contains a base connected to a dielectricgrowth mask of the support, a top comprising a sharp point or a mesahaving a width smaller than a width of the base, and continuously slopedsidewalls connecting the base to the top; and wherein: the firsttemperature is higher than the second temperature; the first pressure islower than the second pressure; and the first V-III ratio is lower thanthe second V-III ratio.
 2. The method of claim 1 wherein: forming thefirst conductivity type III-V semiconductor core or core seed comprisesforming the core using paused CVD comprising alternating growth andnitridation steps; the first V-III ratio during the growth steps inpaused CVD is lower than the second V-III ratio; and a metal organicprecursor flow is paused while a nitrogen containing precursor flow isincreased during the nitridation steps.
 3. The method of claim 1 whereinthe nanopyramid is located over the dielectric growth mask containing anopening.
 4. The method of claim 3 wherein the first conductivity typesemiconductor core or core seed comprises the core which is epitaxiallygrown on a semiconductor buffer layer exposed in the opening such thatthe core protrudes from the opening.
 5. The method of claim 3 whereinthe first conductivity type semiconductor core or core seed comprisesthe core seed which is nucleated on a semiconductor buffer layer exposedin the opening such that the core seed is located in the opening.
 6. Themethod of claim 1 further comprising forming a second layer of thesecond conductivity type semiconductor over the first layer of thesecond conductivity type semiconductor and forming an electrode over thesecond layer of the second conductivity type.
 7. The method of claim 6further comprising forming at least one feature which reduces oreliminates leakage current at tips of the nanopyramids.
 8. The method ofclaim 7 wherein the at least one feature is selected from at least oneof an insulating material tip mask, a hydrogen implanted semiconductortip region having a higher resistivity than semiconductor sidewalls ofthe nanopyramids, a plasma damaged semiconductor tip region having ahigher resistivity than semiconductor sidewalls of the nanopyramids, aflattened tip in truncated pyramid shaped nanopyramids, and the firstconductivity type semiconductor shell having a truncated pyramid shape.9. The method of claim 6 further comprising forming at least one featurewhich reduces or eliminates leakage current at bases of thenanopyramids.
 10. The method of claim 1 wherein: the first temperatureis 1000 to 1200° C. and the second temperature is 800 to 1000° C.; thefirst pressure is 50 to 100 torr and the second pressure is 100 to 500torr; and the first III-V ratio is 5 to 10 and the second V-III ratio is10,000 to 15,000.